IndustrIal PrIme movers
Most industrial processes require objects or substances to be moved from one location to another, or a force to be applied to hold, shape or compress a product. Such activities are performed by prime movers, the workhorses of manufacturing industries.
In many locations all prime movers are electrical. Rotary motions can be provided by simple motors, and linear motion can be obtained from rotary motion by devices such as screw jacks or rack and pinions. Where a pure force or a short linear stroke is required a solenoid may be used (although there are limits to the force that can be obtained by this means).
Electrical devices are not, however, the only means of providing prime movers. Enclosed fluids (both liquids and gases) can also be used to convey energy from one location to another and, consequently, to produce rotary or linear motion or apply a force. Fluid-based systems using liquids as transmission media are called hydraulic systems (from the Greek words hydra for water and aulos for a pipe, descriptions which imply fluids are water although oils are more commonly used). Gas-based systems are called pneumatic systems (from the Greek pneumn for wind or breath). The most common gas is simply compressed air, although nitrogen is occasionally used.
The main advantages and disadvantages of pneumatic or hydraulic sys- tems both arise out of the different characteristics of low-density compress- ible gases and (relatively) high-density incompressible liquids. A pneumatic system, for example, tends to have a ‘softer’ action than a hydraulic system which can be prone to producing noisy and wear-inducing shocks in the pip- ing. A liquid-based hydraulic system, however, can operate at far higher pres- sures than a pneumatic system and, consequently, can be used to provide very large forces.
To compare the various advantages and disadvantages of electrical pneu- matic and hydraulic systems, the following three sections consider how a simple lifting task could be handled by each.
a brIef system comParIson
The task considered is how to lift a load by a distance of about 500 mm. Such tasks are common in manufacturing industries.
With an electrical system we have three basic choices: a solenoid, a DC motor or the ubiquitous workhorse of industry, the AC induction motor. Of these, the solenoid produces a linear stroke directly but its stroke is normally limited to a maximum distance of around 100 mm.
Both DC and AC motors are rotary devices and their outputs need to be converted to linear motion by mechanical devices such as wormscrews or rack and pinions. This presents no real problems; commercial devices are available comprising motor and screw.
The choice of motor depends largely on the speed control requirements. A DC motor fitted with a tacho and driven by a thyristor drive can give excellent speed control, but has high maintenance requirements for brushes and commutator.
An AC motor is virtually maintenance free, but is essentially a fixed-speed de- vice (with speed being determined by number of poles and the supply frequency). Speed can be adjusted with a variable frequency drive, but care needs to be taken to avoid overheating, as most motors are cooled by an internal fan connected di- rectly to the motor shaft. We will assume a fixed speed raise/lower is required, so an AC motor driving a screwjack would seem to be the logical choice.
Neither type of motor can be allowed to stall against an end of travel stop (this is not quite true; specially designed DC motors, featuring good current control on a thyristor drive together with an external cooling fan, can be allowed to stall), so end of travel limits are needed to stop the drive.
We have thus ended up with the system shown in Figure 1.1 comprising a mechanical jack driven by an AC motor controlled by a reversing starter. Aux- iliary equipment comprises two limit switches, and a motor overload protection device. There is no practical load limitation provided screw/gearbox ratio, motor size and contactor rating are correctly calculated.